The present invention relates to demodulation, and more particularly to demodulation of a burstwise signal transmitted over a channel that introduces unknown amplitude and phase changes in the burstwise signal. Even more particularly, the present invention relates to demodulation of burstwise communications signals, such as digital cellular telephone signals and the like, having been transmitted through air and having had unknown changes in amplitude and phase introduced thereinto.
In order to effectively operate in a coherent mode, i.e., a mode characterized by a fixed phase relationship between points on an electromagnetic wave, a receiver must determine phase (and possibly amplitude) distortions introduced by a channel through which a signal traverses in reaching the receiver. For mobile communications-type-channels such as cellular-type channels, in particular, the channel often fades rapidly such that significant changes can occur in phase and amplitude distortions, even over individual signal bursts. Hence, the receiver must repeatedly determine a time-varying estimate of the channel's phase and amplitude distortion characteristics. Nonetheless, coherent operation is highly desirable because capacity of, for example, cellular systems can be increased by about sixty percent (2dB) given constant voice quality.
Determination of a coherent frame-of-reference is frequently based on a portion of each signal burst made up of "known" data, i.e., made up of a sequence of symbols that are known a priori to the receiver. Such "known" data may be for example a synch pattern transmitted at a prescribed position within each signal burst. However, when no such "known" data, or synch pattern, (or an insufficient amount of "known" data) is present within signal bursts of a particular system, the receiver must make this determination "blindly", i.e., without the benefit of a priori knowledge of any significant portion of the signal burst.
In the event that known symbols are not available, one approach is to hypothesize all possible symbols, and to evaluate how well each such hypothesis matches the samples actually received. For example, when signal bursts of a length of 40 symbols and a duration of 2 milliseconds, with each symbol representing two bits of data, are utilized in a "blind" system, the total number of possible transmitted bit sequences per burst is 4.sup.40, i.e., approximately 1.2.times.10.sup.24. For each of these possible sequences, the phase and amplitude distortions of the channel could be estimated, and for each possible sequence, hypothesized data (i.e., the possible sequence) can be passed through the estimated channel. Differences between the hypothesized data having been passed through the estimated channel and the burst actually received can be squared and summed so as to yield a statistic, i.e., a measure of the similarity between the burst actually received and the hypothesized data having been passed through the estimated channel. The estimated channel and hypothesized data that together yield the lowest statistic can then be selected, and the selected estimated channel used to determine an estimate of the burst that was actually transmitted.
Unfortunately, this approach to channel estimation is computationally intractable. Thus, what is needed is an approach to blind demodulation of a coherent signal and more particularly to determining phase and amplitude distortions introduced by a channel through which such signal is transmitted that significantly reduces computational demands as compared to determining a channel estimate for every possible bit sequence for every burst and selecting the channel estimate with the smallest error statistic, while at the same time maintaining a high degree of accuracy in channel estimations.
The present invention advantageously addresses the above and other needs.